This invention relates to touch sensing devices having a plurality of touch sensing elements, each comprising first and second overlapping electrically conductive layers carried on a common support and in which the second electrode is spaced from the first electrode and is adapted to be displaced toward the first electrode in response to a touch input, and to methods of making such devices.
Touch sensing devices of the above kind can be used, for example, as user operable touch switches on electrical appliances or electronic equipment or in a keypad operable by a user's finger or other input element. Touch sensing arrays, comprising a large number of touch sensing elements arranged, for example, in a row and column matrix array, can be used as overlays for the display screen of a CRT, liquid crystal, or other display system providing a user interface through which inputs to the system, such as control commands, can be entered by touching appropriate touch sensing elements in the array. Similarly, an array of touch sensing elements can be used as a graphics tablet or like input device to a computer system enabling information according to graphical representations, such as drawings or handwriting drawn on the array, to be entered by the user. The touch input element in this case can be a hand-held stylus. A high resolution array can also be used, for example, as a fingerprint sensor which detects the position of ridges and valleys in a person's fingerprint.
An example of the above kind of touch sensing device and comprising a two-dimensional array of sensing elements is described in EP-A-0397244 in which each element comprises a first conductive layer that is supported on a substrate of glass and a second conductive layer that is common to all sensing elements and is carried on a flexible film overlying all the elements which is supported in spaced relationship over the first conductive layers by means of appropriately positioned discrete spacer elements disposed between the support and the film which maintain a predetermined gap between the layers. Upon touching, the pressure of the touch input causes the second layer to move into contact with the first layer of a sensing element at the point of touching. In this particular example, the first conductive layer comprises one side of a capacitor which is periodically charged and which is discharged upon the second conductive layer, which is held at a reference potential contacting the first conductive layer thereby enabling the touch input to be detected. The use of a carrier film for the second conductive layer and the need to provide spacing elements at predetermined locations to maintain the conductive layers in a normally spaced relationship both add to the complexity of the construction of such a touch sensing device.
In another known example of a touch sensing device array, a first set of parallel conductive strips is carried on an insulating support and a second set of conductive strips extending transverse to the strips of the first set is carried on a deformable membrane extending over, and spaced from, the first set. In response to input pressure as a result of touching, the membrane is locally deformed to bring a portion of a strip of one set, adjacent the point of touching, into contact with a strip of the other set in the manner of a switch whereby the x-y coordinates of the input can be determined. Instead of relying on electrical contact between the first and second conductive layers of the touch sensing device to detect a touch input, a capacitance sensing approach can be adopted in which a touch input is detected according to a change in the capitance of a touch sensing element resulting from one conductive layer being physically displaced relative to the other conductive layer due to touching. The membrane is typically suspended over the first set of strips by supports positioned around the periphery of the array so that the sets of strips are in close proximity. Consequently, the gap between the two sets of strips may not be uniform over the area of the array, particularly if the array is used horizontally in which case there is a tendency for the membrane to droop, so that erroneous operation can occur not only when capacitance sensing is used but also when contact sensing is used, especially if the array is subjected to vibration or mechanical shocks. Due to the use of a film or membrane separate from the support in the above kinds of touch sensing devices, there is a consequent need for an assembly operation in their manufacture. When used in conjunction with an article, for example as an overlay for a liquid crystal display device, then the assembly operation entailed in providing at least the film or membrane of the touch sensing device complicates manufacture.